The present invention relates to a system for controlling operation timing for use in a mechanical apparatus which is operable under program control, such as by a microcomputer, and more particularly to an operation timing controlling system for the registration mechanisms of electrophotographic copying machines.
The construction and operation of a typical electrophotographic copying machine M operable under program control will be described briefly in connection with FIG. 1.
The copying machine M has two paper cassettes 1 and 2 for supplying either of two sizes of copy paper, and includes a photoconductive drum 3 which is rotatable in the direction of the arrow in the drawing. Upon rotation of the drum 3, the drum surface is subjected to the successive steps of uniform charging by a charger 4, exposure to the image of an original projected thereon at an exposure station Ex by an optical system to be described later, development of the latent image by a developing unit 5, transfer of the toner image by a transfer charger 6 onto copy paper fed in timed relation to the rotation of the drum, removal of charge by an a-c charger 7, and separation of the paper by a pawl 8. The paper separated from the drum is sent by a conveyor belt 9 to a fixing unit having heat rollers 10 by which the toner image is fixed with heat and/or pressure, whereupon the copy is discharged onto a tray 11. The toner remaining on the surface of the photoconductive drum 3 after the separation step is removed by a cleaning unit 12. An eraser 13 removes the residual charge from the drum surface, preparing the drum for a subsequent copying cycle. The optical system comprises an exposure lamp 14, first to fourth reflecting mirrors 15 to 18, and a lens 19. The exposure lamp 14 and the first reflecting mirror 15 are movable together leftward in FIG. 1 at a speed V/M in which V is the speed of travel for making equal size copies, and M is the copying magnification. The second reflecting mirror 16 is similarly movable leftward at one-half the speed of travel of the lamp 14. The reflectors 17 and 18 are stationary.
With copying machines in which two paper cassettes are mounted one above the other as described above, the length of the path extending from one cassette to the transfer station is different from that extending from the other cassette to the transfer station, as shown in FIG. 1. In order to properly position the leading end of the copy paper relative to the toner image on the drum in such a copying machine, the means for transport of the copy paper must be so set as to selectively operate one or the other of feed rollers 20 and 21 depending on which of the cassettes is to supply the paper and also to effect accurate registration by causing the copy paper to be temporarily halted for a period of time suitable to the particular type of paper selected. Many apparatuses have heretofore been proposed or provided for this purpose.
On the other hand, it has been the recent practice to control the operation of such a copying machine M by a digital control system, such as a microcomputer, capable of working out a program and which is incorporated in the machine M.
For such programmed control, the microcomputer functions as a timer for setting predetermined timing intervals based on the data stored in the microcomputer to give signals for controlling the operations of various components of the copying machine. The microcomputer times the sequence of operations on receiving a copying cycle initiating signal.
When a microcomputer is used for controlling the operation of the copying machine, this assures extremely accurate timing control and is also advantageous in that it can be used for setting a wide range of complex timings without making the construction of the copying machine itself complex.
However, because the microcomputer provides control signals in accordance with the data stored therein as described above and, accordingly, because all the copying machines of the same type to be controlled therewith will be controlled by the same timing signals for a given operation, it has been found that this mode of control, if used to achieve the aforementioned registration, gives rise to the following drawback.
In the copying machine, the registration mechanism functions to temporarily stop the paper in the course of the transport thereof and thereafter allows the paper to resume its forward travel in timed relation to the toner image on the surface of the photoconductive drum. To accomplish this, a timing roller 24 is provided which operates in the following manner. The copy paper forwarded, for example by the feed roller 20, comes into contact with the timing roller 24 in its stopped position, whereupon the roller 24 halts the leading end of the paper (see FIG. 1). An unillustrated clutch for driving the roller 24 is thereafter engaged in response to a predetermined timing signal to cause the paper to resume its travel again.
In this arrangement, the timing roller 24 actually starts to move some time after the clutch has received the signal which engages the clutch. This delay time varies from clutch to clutch. Consequently even if the timing signal given by the microcomputer is accurate, the position of the toner image transferred onto the paper differs slightly from machine to machine, causing problems in the actual use of the machine.
However, since the microcomputer produces timing signals in accordance with a predetermined program which cannot be altered easily, it is impossible to check each machine and remedy the possible error in the clutch operation by adjusting the timing with which the timing signal is emitted. This is one of the drawbacks of the microcomputer.
Although the errors involved in such a mechanical operation can not be corrected by adjusting the microcomputer, in order to provide a high quality copying machine, the erroneous operation of the registration mechanism of the copying machine, which appears directly in the copied image, must nevertheless be remedied.